All the events that happened:
- Mio escaped from Westcott
- Mio meets Shinji
- Mio was brought to Takamiya household
- Shinji gave her the name Mio and Mana gave her the Takamiya surname
- Shinji gave her the teddy bear
- Westcott and Elliot are looking for Mio
- Shinji invites Mio on a date
- Mana calls Haruko Homura (Mana's best friend that gave birth to Kotori years later) to give Mio some stylish clothes for her date
- Shinji and Mio went on a date
- Westcott and his men went to chase Mio and Shinji
- Mio and Shinji encountered Elliot but decided to let them go, which led to him betraying DEM and formed Ratatoskr some time later
- Westcott and his men managed to corner Shinji and took Mana into custody (That's why she became a Wizard in the first place by manipulating her memories and experimenting on her, unable to age, so Mana should've been around 40s I think)
- Shinji grabbed Mio and tried to escape again
- Westcott shot Shinji in the chest dead.
- Mio unleashed a space-quake due to her grief, which killed Westcott's men but Westcott himself used teleport magic to escape
- Mio absorbed Shinji's body into her womb to recreate him
- Shinji was later reborn as the Shido Itsuka we know
It goes without saying that Isaac Westcott is the REAL villain of Date A Live and Mio was just a victim. Shinji's death made a grave impact to her life and it made her mad, which led to the events of Kurumi's backstory, and obsession to her goal regardless of how horrible her method is. All for Shinji. As for who Westcott is, keep watching to out.